Melatonin has been shown to affect circadian rhythms in mammals, including humans, and has been used in therapeutic applications such as treating jet lag and some sleep disorders. However, the mechanisms by which melatonin exerts its effects are not well understood. In order to address this issue, the specific aim of the research proposal is to investigate the mechanism by which melatonin resets the circadian pacemaker. Based upon the behavioral effects of melatonin observed in mammals, we hypothesized that melatonin acts to suppress light signals in the suprachiasmatic nucleus (SCN), which is known to be the anatomical substrate for the mammalian circadian clock. Using whole-cell patch clamp and molecular biological techniques this hypothesis will be tested by studying the effects of melatonin on the membrane properties as well as on intracellular effector molecules in SCN neurons. Determining the signal transduction pathways triggered by melatonin is key to defining its function in circadian rhythms.